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Triplet state spectroscopic studies on some 5,10,15,20-tetrakis(methoxyphenyl)porphyrins

Abstract Extensive triplet state spectroscopic investigations were carried out with a series of 5,10,15,20-tetrakis(methoxyphenyl)porphyrins. Triplet absorption spectra, triplet lifetime, triplet quantum yield and quantum yield for singlet oxygen production were determined with different absorption and emission techniques, using the frequency-doubled beam of a Nd:YAG laser. It has been found that these synthetic porphyrins are effective photosensitizers which can be used as model compounds to investigate the theoretical and instrumental aspects of PDT.

MONTE-CARLO MODEL FOR THE HYDROGENATION OF ALKENES ON METAL CATALYST

A Monte‐Carlo model for the simulation of alkene hydrogenation on metallic catalysts has been developed and implemented in Fortran language. We describe the model employed for ethylene hydrogenation on platinum and show the flow chart of the program. Computational characteristics such as number of necessary calculations to reach steady state, running times on different platforms, and effect of the size of the catalyst matrix, are presented. Good correlation between simulated and experimental data was observed. A subroutine allows for visual observation of the reaction. This approach is very useful for obtaining a personal impression of the important factors governing the reaction. By using this example the advantages of Monte‐Carlo simulation to test the level of understanding of catalytic phenomena are discussed. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 396–403, 1998

Absorption and Fluorescence Spectroscopic Study on Complexation of Oxazine 1 Dye by Calix[8]Arenesulfonate

It has been established by combined absorption and fluorescence measurements that the cationic dye Oxazine 1 (OX) and the polyvalent anionic host calix[8]arenesulfonate (SCA8) form two complexes in simultaneous reactions: OX + SCA8 ↔ OX·SCA8 (1) and OX·SCA8 + OX ↔ OX2·SCA8 (2) The equilibrium constants for the two reactions, as functions of the ionic strength (I), and the absorption and fluorescence spectra of the two complex species have been determined by a least-squares fitting method from the experimental data. The variations of the binding constants with the ionic strength could be described on the basis of Debye–Hückel theory. The equilibrium constants are large; their values extrapolated to I = 0 are K(1) = 5.5 × 106 M−1 and K(2) = 4.4 × 105 M−1. The fluorescence of OX undergoes a strong static quenching upon complexation. These results indicate that the complexes are held together by strong electrostatic forces. The addition of non-fluorescent tetramethylammonium chloride to OX–SCA8 mixtures results in a dramatic fluorescent enhancement, which demonstrates the potential applicability of this supramolecular system in fluorescence assays.

A possible construction of a complex chemical reaction network

A procedure is suggested for the construction of chemical reaction networks. We define the kinetic communication as a transfer of atoms or atomic groups between two species and determine all the kinetic communications occurring in the possible mechanism of a complex chemical process. The set of kinetic communications is the basis of the communication matrices resulting in the complete network of the overall reaction.Limiting the consideration for certain types of kinetic communications we obtain the reaction subnetworks and selecting arbitrarily species among those participating in the possible mechanism we introduced the concept of the partial subnetworks which correspond to subsets of the complete network.By the simple analysis of the subnetworks it is easy to obtain the sequence network indicating the pathways via which the selected species are formed in the course of the overall process, by the transfer of chosen atoms or atomic groups.

RECENT APPLICATIONS OF THE KINETIC ISOTOPE METHOD

The kinetics of chemical processes and the theory of simple reactions was developed at the end of the last century. For complex reactions producing highly reactive intermediates that are subsequently consumed, the kinetic treatment is more difficult. Assuming a mechanism that comprises a series of consecutive and parallel elementary steps, we can formulate the corresponding differential equations. Their integration, however, is often impossible. Due to these difficulties, approximate treatments are often applied to the intermediates, such as those based on the Bodenstein stationary concentration principle,’ or the Bodenstein-Semenov principle for degenerate branching chain reactions.’ Though these approximations are useful, their wide application is inhibited hy the lack of analytical data for the intermediates and end-products. Even for thoroughly studied reactions, any additional data bearing on the process generally makes it necessary to formulate a new mechanism and solution of the equation system. This can be illustrated well by several chemical processes discussed in the literature. It became evident in the last decades that there was a need for new methods to avoid the tedious calculations outlined above. Basically, there were two possibilities: (1 ) the detailed and quantitative study of elementary reactions in “artificial” mixtures under experimental conditions where the disturbing effects of other reactions can be excluded and ( 2 ) the in vivo study of elementary steps under “natural” circumstances when the reactions of interest can be investigated separately. An example of the second method is the kinetic application of isotopes.

Solvation and protonation of coumarin 102 in aqueous media: A fluorescence spectroscopic and theoretical study

The ground- and excited-state protonation of Coumarin 102 (C102), a fluorescent probe applied frequently in heterogeneous systems with an aqueous phase, has been studied in aqueous solutions by spectroscopic experiments and theoretical calculations. For the dissociation constant of the protonated form in the ground state, pKa = 1.61 was obtained from the absorption spectra; for the excited-state dissociation constant, pKa* = 2.19 was obtained from the fluorescence spectra. These values were closely reproduced by theoretical calculations via a thermodynamic cycle (the value of pKa* also by calculations via the Förster cycle) using an implicit–explicit solvation model (polarized continuum model + addition of a solvent molecule). The theoretical calculations indicated that (i) in the ground state, C102 occurs primarily as a hydrogen-bonded water complex, with the oxo group as the binding site, (ii) this hydrogen bond becomes stronger upon excitation, and (iii) in the ground state, the amino nitrogen atom is the protonation site, and in the excited state, the carboxy oxygen atom is the protonation site. A comprehensive analysis of fluorescence decay data yielded the values kpr = 3.27 × 10(10) M(–1) s(–1) for the rate constant of the excited-state protonation and kdpr = 2.78 × 10(8) s(–1) for the rate constant of the reverse process (kpr and kdpr were treated as independent parameters). This, considering the relatively long fluorescence lifetimes of neutral C102 (6.02 ns) and its protonated form (3.06 ns) in aqueous media, means that a quasi-equilibrium state of excited-state proton transfer is reached in strongly acidic solutions.

Physico-chemical modeling of the role of free radicals in photodynamic therapy. I. Utilization of quantum yield data of singlet oxygen formation for the study of the interaction between excited photosensitizer and stable free radicals

The measurement of the relative quantum yield of singlet oxygen formation, a simple process without tedious sample deoxygenation, is shown to furnish data on the interaction of the excited photosensitizer with any additive (in this case stable free radicals) given to the sample. The rate constants derived from such measurements are in good agreement with direct determination of the corresponding values.

A Monte Carlo simulation of diffusion-controlled reactions in inhomogeneous systems

Abstract A random-walk model has been developed to treat the kinetics of reactions taking place in spurs at different solute concentrations. The dependence of the molecular yields on solute concentration has been studied using Monte Carlo techniques for simulation of the formation of spurs and of the random movement of the reactive species.

A possible construction of chemical reaction networks

The chemical reaction networks and the sequence networks represent the pathways of a complex chemical process. In order to study the pathways separately the systematization of the elementary processes included in the possible mechanism is inevitable.This systematization was realized by a special procedure based on linear algebraic methods and enabled us to select the corresponding processes from the possible mechanism. The efficiency of the procedure has been illustrated by its application to the liquid phase oxidation of ethylbenzene and the elementary processes have been selected using a computer program.

Quenching of Porphyrin Triplet and Singlet Oxygen by Stable Nitroxide Radicals: Importance of Steric Hindrance

To study the nature of quenching, we used stable nitroxide radicals (most of which contain piperidine or pyrrolidine rings) as quenchers of triplet hematoporphyrin as well as of singlet molecular oxygen. A characteristic feature of quenching triplet porphyrin is a near-diffusion-limited rate constant, whereas the rate constant for quenching singlet oxygen is about three orders of magnitude lower. Accessibility of the nitroxide moiety in radicals was characterized quantitatively based on semi-empirical calculations (at AM1 level), with the use of van der Waals radii of the species. While variation in the rate constant values for quenching triplet porphyrin can be fully explained by the steric hindrance of the neighbouring groups of the nitroxide radical, no such effect can be observed in quenching singlet oxygen.